This invention relates to a piezoelectric loudspeaker which is small in thickness and generates a sound by means of vibrations of a piezoelectric diaphragm.
A moving-coil loudspeaker now widely used restricts various acoustic instruments from being small-sized or small in thickness. The possibility of a thin piezoelectric loudspeaker has recently attracted the public attention.
The thin piezoelectric loudspeaker, as shown in FIG. 1, comprises a diaphragm 2 stretched across a frame 1 and a piezoelectric diaphragm 3 of a bending mode type which is stuck at the central portion of diaphragm 2, the piezoelectric diaphragm 3 comprising a piezoelectric ceramic plate stuck onto a metallic plate. Such piezoelectric loudspeaker has a thin diaphragm 2 and also a thin piezoelectric diaphragm 3, thereby making it possible to generate a sound even in the low frequency band side of audio frequency region.
The piezoelectric loudspeaker having the foregoing construction however, is still insufficient for the sound pressure level and acoustic distortion in comparison with the moving-coil loudspeaker widely used.
After various experiments in order to solve the above problem, we have found that the above piezoelectric loudspeaker, as shown in FIG. 2, is irregular in its bending mode type. As seen from the actual vibration mode shown in FIG. 2, whole the diaphragm 2 and the piezoelectric diaphragm 3 are not vibrated as a single unit. Rather, the peripheral edges (shown as A,B) of the piezoelectric diaphragm 3 are vibrated vertically, resulting in that air is simultaneously pushed out and taken in.
For example, when the peripheral portion of piezoelectric diaphragm 3 is bent downwardly in FIG. 2, air is pushed out at area .alpha. shown by oblique line and taken in at area .beta. shown by oblique line done so, whereby pushing out and taking in of the air interfere mutually so as to cause a low sound pressure level. Also differences in volume of an input signal change the vertical movement of fulca A and B resulting in degree of interference, thereby causing the acoustic distortion.
Thus, the conventional piezoelectric loudspeaker is low in the sound pressure level and larger in the acoustic distortion, which is inferior in performance and hinders practical use.